Sawmills commonly saw logs or semifinished pieces of lumber, called cants, using multiple bandmills to make several sawlines in one pass, with the path of the moving log or cant oriented approximately parallel to its longitudinal centerline. The multiple bandmills presently used in sawmills commonly consist of one or two pairs of single bandmills.
A single bandmill consists of two large wheels of equal size, usually 4 feet to 10 feet in diameter with a bandsaw blade mounted thereon. One wheel, the bottom wheel, is usually mounted below the floor level of the sawmill. This wheel is mounted on an arbor which is turned by a motor. The second wheel, the top wheel, is mounted on an arbor which, in turn, is mounted on a columnular framework above the bottom wheel. Both wheels are usually mounted vehicle, with the radial direction of the wheel perpendicular to the direction of log or cant travel. The bandsaw blade, consisting of a flat band of steel made into an endless loop with sawteeth on one edge, is mounted on the two wheels. The rotation of the bottom wheel moves the bandsaw blade by frictional force. The moving bandsaw blade causes the top wheel to turn, also by friction. The path of travel of any tooth on the bandsaw blade, then, is described by semicircles at the bottom of the bottom wheel and the top of the top wheel joined by straight lines between the ends of the semicircles.
Because the bandsaw blade moves around the two wheels, the direction of sawblade travel is downward on one side and upward on the other. The actual cutting operation is performed on the side where the sawblade is traveling in the downward direction.
The sides of the bandmill are defined as the two sides where the wheels are viewed as a circle. The front and back of the bandmill are the two sides where the bandmill wheels are seen on edge, with the front being the side on which the bandsaw blade does the cutting.
Two or more single bandmills arranged to operate together as a unit are called a multiple bandmill. When two bandmills are arranged opposite each other with the fronts facing each other, the pair is called in a twin bandmill. Thus, when looking at the pair of bandmills, they appear to be mirror images of each other. The log or cant to be sawn is transported past the saws in a longitudinal direction with the center of the log or cant approximately on the longitudinal centerline of the unit. The twin bandmill, then, can make two sawlines in the log or cant at the same time, with the two sawlines generally being on opposites sides of the log or cant longitudinal centerline. The two bandsaw blades and the resulting sawlines in the log or cant are parallel. The cutting edges of the two bandsaw blades are in the same vertical plane, perpendicular to the direction of log or cant travel.
Commonly, two pairs of twin bandmills are arranged with one pair ahead of theother, so four parallel sawlines can be made at almost the same time. This arrangement is called a quad bandmill. The two pairs of bandmills comprising the quad bandmill are separated longitudinally along the sawing path by typically one and one-half feet to as much as six or eight feet. The longitudinal separation between the two pairs of bandmills is necessary to accommodate the width of the wheels, the supporting columns, and the arbors on which the wheels are mounted.
When sawing a log with a quad bandmill, the first pair of saws cutting the log makes the two sawlines closest to the outside of the log, while the second pair makes the two sawlines closest to the center of the log. It is this longitudinal separation of the two pairs of bandmills comprising the quad bandmill which leads to some of the inaccuracy in the dimensions of the sawn lumber manufactured on the quad bandmill, for reasons described below. This inaccuracy is prevented by the present invention.
As a tree grows, tension stresses develop in the newly formed xylem, see for instance, Forest Products Journal 33(3) 10, 1983. With the formation of subsequent annual growth rings, the area of tension stress expands and induces compression stresses in the center of the tree. The result is a continuum of stresses, called growth stresses, radially across the tree, with tension stresses at the outside, decreasing to a neutral zone approximately one-third of the way to the center of the tree, and changing to compression stresses which are greatest at the pith. When a log from the tree is sawn into lumber, the cutting of the wood fibers by the saw relieves some of these stresses. This stress relief causes lumber sawn from the log to warp while the unsawn portion of the log moves with respect to its initial position in the sawing system. When logs are sawn using a quad bandmill, the initial two cuts by the first pair of bandmills relieve some of the stresses, thus allowing the remainder of the log to move from its original position prior to sawing by the second pair of bandmills. The growth stresses are not necessarily uniform within a given tree, and vary in both magnitude and location from tree to tree. Thus, the amount of log movement in sawing varies erratically and unpredictably.
The quad bandmills are set up so the sawlines from all four bandmills are parallel and spaced apart by the target thickness of the lumber. Since the log moves unpredictably relative to the saws, the dimensions of the lumber manufactured from it will also vary. To account for this variation, additional thickness is added to the lumber target size so no lumber is manufactured too thin. In this manner, undersized lumber is prevented, but the addition of thickness means some lumber is manufactured oversized, which wastes wood fiber and reduces lumber yield from each log.
If all four sawlines were made at the same longitudinal location at one time, growth stress relief would still occur, but since sawing would be completed, the resulting log movement would have no effect on the accuracy of the lumber. However, this is impossible to accomplish with existing quad bandmills because of the distance required between the pairs of bandmills.